This invention relates to a method and apparatus for aligning a plurality of objects, e.g., a wafer and mask, overlapping closely each other by measuring the relative displacement of the objects, and the invention particularly relates to a method and apparatus for alignment suitable for a proximity aligner system such as an X-ray exposure system in order to expose both the mask and the wafer.
Conventionally, relative positioning between the mask and wafer has been carried out by detecting positioning marks using a microscope, as described in Japanese Patent Publication No. 57-42971 or U.S. patent application Ser. No. 580,709 (EPC 841056). However, in this system, where the objective lens of the microscope is located upright over the plane of the mask and wafer, when it is intended to transfer a circuit pattern from the mask to the wafer by the X-ray exposure system described in U.S. Pat. No. 4,403,336, for example, the optical system for mark detection interferes with the exposure light, e.g., X-ray. Therefore, it is necessary for the microscope to have its objective lens kept out of the exposing optical system, and this time consuming operation results in a deteriorated throughput. For example, among latest LSI devices which are going to have a line width of smaller than 1 .mu.m, the X-ray aligner which is a typical proximity aligner is intended to deal with a line width smaller than 0.8 .mu.m. On this account, the alignment accuracy as fine as 0.3 .mu.m is now required.
Methods of alignment detection include magnification of the alignment pattern, scanning of the alignment pattern using a laser beam and alignment method using a diffraction grating. The first-mentioned method is that alignment patterns of the mask and wafer are magnified by the objective lens and focused on the imaging element, the relative displacement of both patterns is evaluated through the signal processing, and the wafer is moved relative to the mask so that the relative displacement does not exist. The above-mentioned 0.3 .mu.m alignment accuracy includes the mechanical error of the aligner and the dimensional error of the mask and wafer, and therefore the pure alignment accuracy required is as high as 0.1 .mu.m.
To cope with this matter, there has been proposed an alignment method, as described in Japanese Patent Publication No. 57-42971 (1982) for example, in which an objective lens with a smaller focal depth is used to obtain a larger lens aperture so that a circuit pattern with a poor contrast can be imaged using much amount of light. In this case, however, the lens needs to have a magnification of 40 to 60 and an NA of 0.5 or more, as well as a larger aperture and smaller operating distance, and therefore there is a possibility of interference of the objective lens with the exposing X-ray when the alignment pattern is located within the exposure area. This has necessitated the retraction of the objective lens set at each exposure, which has impaired the throughput and disallowed the alignment detection concurrently with exposure.
In the conventional mask-wafer positioning method and apparatus, the fresnel zone marks made on the mask and wafer are illuminated oblique from outside of the exposure area into the exposure area so that the coherent light diffracted in the fresnel zone focuses in the exposure area, and therefore the objective lens for detecting the focal position must be located in the exposure area. Accordingly, the focal point in the fresnel zone is detected by the objective lens and, after alignment of the wafer to the mask, the objective lens must be retracted. The conventional exposing method has employed a single exposing operation for the entire surface of a wafer, and a slight overhead time has not been a serious matter. A positioning method of this category is disclosed in U.S. patent application Ser. No. 139,544.
In the case of the step-and-repeat method which is intended to transfer a fine pattern with less than 1 .mu.m line width, the alignment and exposing operations are repeated more than ten times for each wafer, and the reduction of the alignment time is an urgent request. An associated problem is the deterioration of the mask-wafer alignment accuracy due to the vibration when the objective lens is moved following each mask-wafer alignment operation.